Wide frequency response line scan magnetic deflector

ABSTRACT

A magnetic deflector for an electron beam is disclosed. The deflector is characterized by a frequency response of several megahertz and a linear deflection. The wide frequency response and linearity are achieved by utilizing particularly shaped pole pieces within the electron beam gun. In additional embodiments, combinations of deflecting elements are disclosed.

United States Patent Hughes [4 1 June 27, 1972 [54] WIDE FREQUENCYRESPONSE LINE 2,909,688 10/1959 Archard .335/210 x SCAN MAGNETICDEFLECTOR 3,197,678 7/1965 Primas.... ..335/209 3,482,136 12/1969Herrera ..250/49.5 C X [72] inventor: William C. Hughes, Scotia, NY.

73 Ass nee: General Electric Com Examiner ceorge Hams 1 AttmeyRichard R.Brajnard, Paul A. Frank, Charles T. Filed: March 1, 1971 Watts, Frank L.Neuhauser, Oscar Bl Waddelland Joseph B. 21 Appl. 190.; 119,584 Fmma[57] ABSTRACT US. Cl. ..335/210, 313/79, A magnetic deflector for anelectron beam is disclosed The [58] Fieid Search /49 C deflector ischaracterized by a frequency response of several "250/49 13/73 76megahertz and a linear deflection. The wide frequency response andlinearity are achieved by utilizing particularly shaped pole pieceswithin the electron beam gun. in additional [56] References cuedembodiments, combinations of deflecting elements are dis- UNITED STATESPATENTS Closed- 2,777,958 1/1957 Poole ..250/49.5 D Chins, 8 DrawingFigures PATENTED'JUNZ'I'ISR SHEET'10F3 FIG H/S ATTORNEY WIDE FREQUENCYRESPONSE LINE SCAN MAGNETIC DEFLECTOR THE DISCLOSURE This inventionrelates to magnetic deflection devices, and, in particular, to magneticdeflection devices for use in electron beam recording guns.

In electron beam devices, magnetic deflection is preferred toelectrostatic deflection for several reasons. Magnetic deflection allowswider deflection angles with less spot degradation (spread ordeformation). Since magnetic deflection is current dependent, it ismorecompatible with semiconductor circuitry. Further, since the magneticfield required for a given deflection does not increase as rapidly withbeam voltage as the electrostatic field required, it is more suited tohigh voltage guns.

A swept electron beam has been used to record on a variety of media,such as thermoplastic, photographic and magnetic films. In thisapplication, the beam is used to scan a moving tape, generally alongonly one axis. It is generally required that the beam produce a verysmall spot (e. g. less than 6 microns in diameter) and have a highcurrent density. Also, it is advantageous to place the final lens of thegun as closely as possi ble to the recording media, which allows minimaldistance for deflection. In addition, it is desirable to have a highrepetition rate.

These combined specifications require an electron gun to have widedeflection angles, wide frequency response, high voltage, linear sweepand low spot degradation.

While these requirements would seemingly be fulfilled by a magneticdeflection system, such systems have been little used in this type ofapplication because of the non-linearity in the deflection vs. currentcharacteristic of the magnetic deflection system. Also, it is difiicultto obtain a wide frequency response with conventional magneticdeflection systems. Further, there is often a problem in fitting aconventional deflection yoke into the mechanical design of the gun.

Thus, until now, one had the choice of accepting the performanceavailable with electrostatic deflection or utilizing electroniccorrection in the deflection amplifier to correct for the non-lineardeflection characteristic of a magnetic deflector.

The foregoing difficulties are compounded when one desires to usemultiple, cascaded deflection systems. This would occur when it isdesired that the beam strike the target at a particular angle across theentire trace; for example, normal to the plane, of a flat target. Onedeflector deflects the beam to the desired location, a second deflectorstraightens" the beam to the desired angle. As another example, when thedeflector is to be used with an electron lens and the lens must be closeto the target, it has been found that redirecting the beam through thenodal point of the lens enables wide deflection angles, despite theclose proximity of the lens to the target.

In view of the foregoing, it is therefore an object of the presentinvention to provide a magnetic deflection system having wide frequencyresponse.

It is a further object of the present invention to provide a magneticdeflection system for a frequency response extending to at least 9 Mhz.

It is another object of the present invention to provide a magneticdeflection system having a wide deflection angle and a linear sweep.

It is a further object of the present invention to provide a magneticdeflection system for use with electron beams of high current density.

It is a further object of the present invention to provide a magneticdeflection system having a low spot degradation.

It is another object of the present invention to provide a magneticdeflection system comprising cascaded deflection elements providing widedeflection angles and a linear sweep.

The foregoing objects are achieved in the present invention pole piecesin close proximity to the electron beam. The shaping of the pole piecesis a function of the positioning of the deflection system relative tothe target. This function produces a family of curves or shapes for thepole pieces depending upon the distance to the target. In an alternativeembodiment of the present invention, electrostatic deflecting plates arepositioned between the shaped, magnetic pole pieces and utilized toextend the high frequency response'of the deflection system.

A more complete understanding of the present invention may be obtainedby a consideration of the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 illustrates a conventional magnetic deflection system and theparameters utilized in determining the shaping of the pole pieces.

FIG. 2 is a diagram useful in explaining the present invention.

FIG. 3 illustrates a graphical representation of the shape of differentpole pieces depending upon the distance between the deflection systemand the target.

FIG. 4 illustrates a crossvsectional view of a magnetic deflectionsystem in accordance with the present invention.

FIG. 5 illustrates an end view of a magnetic deflection system inaccordance with the present invention.

FIG. 6 illustrates a graphical representation of the shaping of the polepieces in accordance with the distance of the magnetic deflection systemfrom the target.

FIG. 7 illustrates a graphical representation of a pole piece for themagnetic deflection system when cascaded with other deflecting elements.

FIG. 8 illustrates the combination of various shaped pole pieces used todeflect the electron beam to the nodal point of an electron lens.

Referring to FIG. 1, there is illustrated the basic elements utilized ina magnetic deflection system. A magnetic deflection system generallycomprises a magnetic core 10 having a plurality of turns of wire 1 Iwound thereon and magnetic pole pieces or faces 12 for establishing auniform magnetic field across the gap 3 defined by pole pieces 12. Theamount of deflection is determined by the magnetic field, which in turnis dependent upon the current through coil 11 and the number of turns ofcoil 11. The amount of deflection is also dependent upon the length L ofpole pieces 12.

As an electron beam enters the field established between pole pieces 12,the electrons of the beam follow a curved path such as path 14. With noapplied magnetic field, the electrons are not deflected and passdirectly through the gap along path 15. The amount of deflection d ofthe electron beam upon intercepting target 13 is dependent upon thedistance S between the deflecting elements and target 13 and the lengthof the plates L as well as the strength of the magnetic field.Specifically, the deflection sensitivity (in meters per ampere) is givenby the following relationship:

Deflection sensitivity (e t/mv) [(S0.5L) LN/g] 1) A more completeunderstanding of the nonlinearities involved in a magnetic deflectionsystem may best be understood by considering FIG. 2 in which there isillustrated a simplified cross-sectional diagram of the magneticdeflecting system of FIG. 1. The amount of deflection of the electronbeam is determined by the length of radius R which in turn is determinedby the strength of the magnetic field. The angle through which radius Rrotates is determined by the length L of pole pieces 12.

In the basic deflection system illustrated in FIG. 2 are a number ofareas which serve to make the deflection vs. input currentcharacteristic of the deflection system nonlinear. A first of theseresides in the nature of the beam path between pole pieces 12. For zerodeflection, the length of the beam path, and hence the length of time agiven electron is traveling between pole pieces 12, is at a minimumsince the length of the beam path between pole pieces 12 is equal to L.Since the wherein there is provided a magnetic deflector having shapedpole pieces are rectangular and the beam path is the arc of a circle ofradius R, the length of the beam path between pole pieces 12 is greaterduring deflection than it was before, thereby increasing the anglethrough which the beam is deflected. Thus, for small radii the beam isdeflected relatively more than it should be.

In addition, the amount of deflection of the electron beam is determinedin part by the angle of deflection and distance S-L. A portion of thedeflection, Y, is determined by the displacement of the beam as it isdeflected between pole pieces 12. The amount of displacement (d-Y) is atangential function of angle 6. Further, displacement Y is a non-linearfunction of 6.

For zero deflection, an electron traveling through pole pieces 12 willpass through origin 0 along beam path 15 and strike target 13 at point40. For a given amount of deflection, 0, the electron beam is curvedwhile passing between pole pieces 12 and exits at an angle 0 from thenormal to the target, follows path 14 and strikes target 13 at point 41.

In addition to correcting the above enumerated nonlinearities, thedeflecting system of the present invention further enables the highrepetition rates necessary in a line scanning system. In general, a widefrequency response is obtained by a proper balance between theinductance exhibited by the deflection system with the capacitance ofthe windings.

The correction for nonlinearities is obtained by the shaping of the polepieces in accordance with the deflection distance S, as will be morefully enumerated hereinafter.

In order to achieve wide frequency response, it is desirable to make thepole pieces as small and closely spaced as possible, e.g. less than beamdiameters apart. This reduction in size will result in smaller shuntcapacitance across the coil which will further increase theself-resonant frequency of the deflector. The reduction in size producesthe advantage of enabling the deflector to be placed inside the vacuumenvelope containing the electron beam generating means. Presentlyavailable ferrite materials for the core are compatible with the vacuumenvironment and permit a frequency response of several Mhz.

FIG. 3 illustrates a family of curves for different values of Sillustrating the corrective shape for pole pieces 12. For the deflectionsystem illustrated in FIG. 1, that is, with rectangular deflectionplates, the amount of deflection is given by the following equation:

As discussed above, for greater angles of deflection, that is, smallerR, the deflection is greater than it should be for a given radius. Tolinearize the deflector, a way must be found to decrease the deflectionsensitivity at larger deflection values. This can be done by shaping thepole pieces so that at wider deflection angles the beam leaves thedeflection field sooner. In other words, the pole pieces are shaped as afunction of the deflection so that at smaller radii less deflection isproduced.

When fringe fields are neglected, it can be shown that the desired shapefor the pole pieces is given by the following equation:

[(S 4S+ 2) X +(2S S) X+S*S+l/4]% (4) This equation has been normalizedby dividing by L, the length of the plate on axis; i.e. at S l the polepiece is just touching or is very close to the target. The equationholds for 0 g X 1. FIG. 3 illustrates a family of curves for S l and S wand for three values in between. The above equation does not becomeundefined as S as may be seen by inspection of the equation using thebinomial theorm to expand the square root term.

FIG. 4 illustrates a cross-sectional view of a magnetic deflector inaccordance with the present invention. The curves of FIG.- 3 representonly the upper half of pole piece 20 in FIG. 4. The lower half of polepiece 20 is the mirror image of the upper half. Since relatively largedeflection angles (greater than 100) are not being utilized, the entirecurve from FIG. 3 need not be used. Thus, the pole pieces in FIG. 4include straight edge portions 25. The straight edge portionsapproximate the exact pole shape and do not affect the performance ofthe deflector since large deflection angles are not being utilized.

If it is desired to raise the upper frequency limit of the deflectionsystem, beyond the relatively high upper limit already obtained by thepresent invention, electrostatic deflection plates 21 can be insertedbetween magnetic pole pieces 20. In operation, the electrostaticdeflection plates would be activated only at the very high repetitionrate frequencies. At lower repetition rates they would not participatein the deflection.

The response curves for the electrostatic and the magnetic deflectionsystems would be arranged so as to complement one another. That is, asthe frequency response of the magnetic deflection system falls off, theresponse of the electrostatic deflection system would be increased. Thiscan be readily accomplished by suitable frequency filtering in theamplifier driving the electrostatic deflectors.

In order to minimize any disturbance caused in the magnetic field by theintroduction of the electrostatic deflection plates, it is preferredthat deflection plates 21 be formed as a thin layer or film ofelectrically conductive material on nonmagnetic substrate 22. It isunderstood, of course, that other electrostatic deflection plates may beutilized provided they do not distort the magnetic deflection field.

FIG. 5 illustrates an end view of the combined electrostatic andmagnetic deflection systems illustrated in FIG. 4 and shows the relativepositioning of the magnetic and electrostatic deflection elements 20 and21 respectively.

When it is desired to have the beam intercept the target at a particularangle, a pair of magnetic deflection plates can be utilized havingshaped pole pieces as illustrated in FIG. 6. For the first pole piece,whose shape is similar to that illustrated in FIG. 3, the left handleading edge 61, nearest the source of the electron beam, isperpendicular to the axis of the deflection system. The trailing edge ofthe first magnetic deflection system has a shaped configurationdescribed by the following equation:

A comparison of this equation with equation (4) will reveal a slightdifference to allow for the second deflection system. The net effect ofthe first deflection system is the same as that for FIG. 3, however, theelectron beam exits the pole pieces sooner at greater deflection anglesthan it would if the pole pieces were simply rectangular. However, inthis embodiment, the first deflection system alone is not fullycorrected.

The second deflection system has its leading edge represented by afamily of curves for various values of S, the distance in whichdeflection takes place. The trailing edge of the second magneticdeflection pole piece 62 is perpendicular to the axis of the deflectionsystem.

When the combined magnetic deflection systems illustrated in FIGS. 6 areutilized, the deflection distance S is then the length from the leadingedge 61 of the first magnetic deflection system to trailing edge 62 ofthe second magnetic deflection system, since it is only over thisinterval that the electron beam is deflected. The distance S as shown inFIG. 6 should not be construed as indicating that trailing edge 62 isneces sarily immediately adjacent the target. The shape of the leadingedge of the second magnetic deflection system is obtained in accordancewith the following equation:

It is important to note with respect to the second magnetic deflectionsystem how the value X is measured, that is from right to left ratherthan from left to right.

In operation, a magnetic deflection system in accordance with FIG. 6will displace an electron beam a predetermined amount from the zerodeflection path followed by the beam and will cause the beam to strike atarget at a predetermined angle, for example, normal to the plane of thetarget, assuming the target is perpendicular to the axis of thedeflection system. Deflection is obtained by varying the current throughwindings l l and hence varying the radius R through which the electronbeam is deflected in the first deflector. The second deflector also hasa variable current flowing in the windings thereof so as to deflect thebeam through an appropriate radius curve to bring the beam back to adirection that is perpendicular to the plane of the target and parallelto the undeflected beam path. As with the shaped pole piece illustratedin FIG. 3, the deflection d produced by the deflection systems of FIG. 6is linearly proportional to the input current.

FIG. 7 illustrates a shaped pole piece for bringing parallel butseparated electron beams together at a point on the extended axis of thedeflection system. As before, the amount of deflection is linearlyproportional to the input current. The proper shape for these deflectionplates is obtained from the following equations:

In FIG. 7, the plate shape for S=3 and D=2 is illustrated. Utilizing apole piece as illustrated in FIG. 7 in a magnetic deflection system, aplurality of parallel electron beams perpendicular to and incident uponleading edge 71 will be deflected by pole piece 70 and exit pasttrailing edge 72 at such an angle that all of the beams will interceptthe axis of the deflection system, if extended, at a point a distance S3 from the leading edge. Such a deflector would find utility, forexample, in conjunction with an electron lens in which it is desirablethat the electron beam pass through the nodal point of the lens. Byutilizing a magnetic deflector as illustrated in FIG. 7, the nodal pointof the electron lens would be placed at S 3 on the axis of the magneticdeflection system.

FIG. 8 illustrates one embodiment of the present invention in which allthree types of magnetic deflectors are utilized. The system of FIG. 8utilizing magnetic deflectors in conjunction with an electron lensenables the electron lens to be placed very close to the target andprovide wide deflection angles in the very short distance available.

An electron .beam entering the deflection systems from the left will bedeflected by first magnetic deflector 81 through an angle such that, inconjunction with second magnetic deflector 82, the electron beam isdisplaced a distance d that is linearly proportional to the inputcurrent. Second magnetic deflector 82 realigns the electron beam to adirection that is parallel to the axis of the deflection system. Thirdmagnetic deflector 83 then redirects the beam to the nodal point ofelectron lens 84. Note in this combination that trailing edge 62 of thesecond magnetic deflector is coincident with leading edge 71 of thethird magnetic deflector. This need not be the case, however. Second andthird magnetic deflectors can be separated provided that there are nostray fields to divert the electron beam from its course parallel to theaxis of the deflection system.

There has thus been described a magnetic deflection system having alinear response and a wide frequency range. The first magnetic deflectoralone can provide a deflection linearly proportional to input currentor, with a slightly different shape, in combination with a secondmagnetic deflector provide a deflection that is linearly proportional toinput current and, in addition, reorients the electron beam to apredetermined direction independent of the amount of deflection. Inaddition, a third shaped pole piece has been described that can beutilized to linearly redirecting parallel electron beams to apredetermined point. In addition, it has been shown how the already highfrequency response of a magnetic deflector in accordance with thepresent invention can be extended through the use of an electrostaticdeflection means that is activated in a complementary fashion to thefrequency response of the magnetic deflector. In a magnetic deflectorbuilt in accordance with the present invention, specifically inaccordance with FIG. 3 and in which S 1.31, the measured inductance ofthe deflector with 1 inch leads was 4 rnicrohenries and theself-resonant frequency was about Mhz. Measurements of deflectionlinearity indicate that the deflection is a linear function of inputcurrent to better than 0.6 percent. For

a magnetic deflection system in accordance with FIG. 1, that is, withrectangular pole pieces, the non-linearity would have been at least 2.4percent.

Having thus described the invention it will be apparent to those skilledin the art that various modifications can be made within the spirit andscope of the present invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A magnetic deflection system for deflecting an electron beamcomprising:

a magnetic core, first and second pole pieces magnetically coupled tosaid core and a coil of wire wound on said core;

said first and second pole pieces being positioned on opposite sides ofsaid electron beam and in close proximity thereto for controlling thedeflection of said electron beam by a magnetic field inducedtherebetween by current through said coil;

said first and second pole pieces being shaped as a function of thedistance from the deflection system to a target to linearize thedeflection of said electron beam as a function of current through saidcoil.

2. A magnetic deflection system as set forth in claim 1 wherein saidfirst and second pole pieces are separated by less than 10 times thediameter of said electron beam.

3. A magnetic deflection system as set forth in claim 1 wherein theshaping of said first and second pole pieces is such that the electronbeam exits the region between said pole pieces sooner at greaterdeflection angles than it would if the pole pieces were rectangular.

4. A magnetic deflection system as set forth in claim 1, wherein theself-resonant frequency of said coil and pole pieces is greater than 50Mhz.

5. A magnetic deflection system as set forth in claim 1 wherein saidfirst and second pole pieces are shaped in accordance with the curve andgenerated by the equation:

Y =X +SX+S- [(S 4S+2) X"+(2S S)XS+ 1 1/2 wherein X and Yare coordinatesof the pole piece and S is the distance in the X direction in which thebeam must be deflected.

6. A magnetic deflection system as set forth in claim 1 furthercomprising:

electrostatic deflection means positioned between said first and secondpole pieces to assist in deflecting said electron beam in the same planeas it is deflected by said first and second pole pieces; said assistancecomplementing the frequency response of said core, coil and pole piecesto extend the frequency response of said deflection system to higherfrequencies.

7. A magnetic deflection system as set forth in claim 6 wherein saidelectrostatic deflection means comprises conductive films on anon-magnetic substrate positioned between said pole pieces.

8. A magnetic deflection system for deflecting an electron beamcomprising:

a plurality of magnetic deflection elements arranged in cascadingfashion along the path followed by an undeflected electron beam;

each of said plurality of magnetic deflection elements comprising amagnetic core, first and second pole pieces magnetically coupled to saidcore and a coil of wire wound on said core;

said first and second pole pieces of each magnetic deflection elementsbeing parallel and positioned on opposite sides of said electron beamand in close proximity thereto for controlling the deflection of saidelectron beam by a magnetic field induced between each of said first andsecond pole pieces by current through said coil; each of said first andsecond pole pieces being shaped as a function of the amount ofdeflection to linearize the deflection of said beam. 9. A magneticdeflection system as set forth in claim 8 comprising first and secondmagnetic deflection elements; the first element deflecting said electronbeam a predetermined amount, the second element aligning the electronbeam to a path parallel to and displaced from the electron beam path

1. A magnetic deflection system for deflecting an electron beamcomprising: a magnetic core, first and second pole pieces magneticallycoupled to said core and a coil of wire wound on said core; said firstand second pole pieces being positioned on opposite sides of saidelectron beam and in close proximity thereto for controlling thedeflection of said electron beam by a magnetic field inducedtherebetween by current through said coil; said first and second polepieces being shaped as a function of the distance from the deflectionsystem to a target to linearize the deflection of said electron beam asa function of current throUgh said coil.
 2. A magnetic deflection systemas set forth in claim 1 wherein said first and second pole pieces areseparated by less than 10 times the diameter of said electron beam.
 3. Amagnetic deflection system as set forth in claim 1 wherein the shapingof said first and second pole pieces is such that the electron beamexits the region between said pole pieces sooner at greater deflectionangles than it would if the pole pieces were rectangular.
 4. A magneticdeflection system as set forth in claim 1, wherein the self-resonantfrequency of said coil and pole pieces is greater than 50 Mhz.
 5. Amagnetic deflection system as set forth in claim 1 wherein said firstand second pole pieces are shaped in accordance with the curve andgenerated by the equation: Y2 -X2 + SX + S - 1/2 - ((S2 - 4S+2) X2 +(2S2 - S)X-S+ 1/4 )1/2 wherein X and Y are coordinates of the pole pieceand S is the distance in the X direction in which the beam must bedeflected.
 6. A magnetic deflection system as set forth in claim 1further comprising: electrostatic deflection means positioned betweensaid first and second pole pieces to assist in deflecting said electronbeam in the same plane as it is deflected by said first and second polepieces; said assistance complementing the frequency response of saidcore, coil and pole pieces to extend the frequency response of saiddeflection system to higher frequencies.
 7. A magnetic deflection systemas set forth in claim 6 wherein said electrostatic deflection meanscomprises conductive films on a non-magnetic substrate positionedbetween said pole pieces.
 8. A magnetic deflection system for deflectingan electron beam comprising: a plurality of magnetic deflection elementsarranged in cascading fashion along the path followed by an undeflectedelectron beam; each of said plurality of magnetic deflection elementscomprising a magnetic core, first and second pole pieces magneticallycoupled to said core and a coil of wire wound on said core; said firstand second pole pieces of each magnetic deflection elements beingparallel and positioned on opposite sides of said electron beam and inclose proximity thereto for controlling the deflection of said electronbeam by a magnetic field induced between each of said first and secondpole pieces by current through said coil; each of said first and secondpole pieces being shaped as a function of the amount of deflection tolinearize the deflection of said beam.
 9. A magnetic deflection systemas set forth in claim 8 comprising first and second magnetic deflectionelements; the first element deflecting said electron beam apredetermined amount, the second element aligning the electron beam to apath parallel to and displaced from the electron beam path prior todefection.
 10. A magnetic deflection system as set forth in claim 8comprising first and second magnetic deflection elements, the firstelement deflecting said electron beam a predetermined amount, the secondelement focusing the deflected beam at a point over the range ofdeflections of said first deflection element.